text: Owing to the decreased electron density in the aromatic system, electrophilic substitutions are suppressed in pyridine and its derivatives. Friedel–Crafts alkylation or acylation, usually fail for pyridine because they lead only to the addition at the nitrogen atom. Substitutions usually occur at the 3-position, which is the most electron-rich carbon atom in the ring and is, therefore, more susceptible to an electrophilic addition.
  Direct nitration of pyridine is sluggish. Pyridine derivatives wherein the nitrogen atom is screened sterically and/or electronically can be obtained by nitration with nitronium tetrafluoroborate (NOBF). In this way, 3-nitropyridine can be obtained via the synthesis of 2,6-dibromopyridine followed by nitration and debromination.
  Sulfonation of pyridine is even more difficult than nitration.   However, pyridine-3-sulfonic acid can be obtained. Reaction with the SO group also facilitates addition of sulfur to the nitrogen atom, especially in the presence of a mercury(II) sulfate catalyst.
  In contrast to the sluggish nitrations and sulfonations, the bromination and chlorination of pyridine proceed well.